It is customary in drilling oilwells through offshore rigs to drive an exterior conductor pipe, which is a string made up of segments of large diameter exterior pipe, to within the soil strata beneath an over water layer.
In general an offshore rig, such as a jack-up rig, will have a 60 foot air gap between the hull and the surface of the water. Above the hull there will be an additional approximately 30 foot distance to the working level of the rotary table. Beneath the water will be found loose, unconsolidated, generally sedimentary soils; at a greater depth will be found a harder, consolidated soil structure.
It is necessary to provide a continuous, cylindrical or casing structure to form the outer bore of an oilwell being drilled beneath the rotary table. The bore must be resistant to pressure and must support fluid flow, since an annulus formed by the casing structure is the containing region for reverse flow of drilling fluids and for containing any downhole kick pressures which are exterior to the drill string.
As a result, current technology drives a continuous string of external conductor pipe to define such an exterior bore. The conductor pipe is driven from a point immediately below the rotary to form a continuous, fluidly connected, pressure resisting string to a point deep enough within the consolidated soil under layer so as to resist washing out and blowout effects around the bottom of the conductor pipe. This typically involves at least 100 feet of soild penetration by cylindrical conductor pipe to set up a proper conductor string.
Conductor pipe is normally driven by an enclosed hammer in the manner of pile driving. However, since the conductor pipe is intended to form a cylindrical bore having a generally high tolerance diameter, within which is contained the oilwell casing and the tubular members of the drillstring, it is necessary that the conductor pipe remain a hollow cylinder and that it retain a circular cross section under the force of the driving. It is likewise important that the conductor pipe be driven in as straight a line as possible, since the geometry of the conductor pipe defines the center line of the oilwell bore and guides and directs the drillstring during subsequent drilling operations.
Current conductor pipe driving technology, like that for driving hollow cylindrical pilings for industrial construction, is a time consuming and expensive process. The procedure is complicated by several factors.
Viscous friction on the inner and outside circumference of the piling causes increasing resistance to penetration as the conductor pipe is driven deeper in the soil.
The necessity of retaining a cylindrical cross-section against the resistance encountered by the leading edge of the conductor pipe as it is driven by hammer blows, especially in dense, hard or irregular strata, currently results in the leading edge being chamfered with an outwardly inclined bevel section. The primary purpose of this chamfer is to avoid forces which would tend to collapse or crimp the edge of the conductor pipe, rendering the conductor string useless.
As a result of this outward bevel, essentially all of the soil that is displaced by the driving of the conductor pipe is compressed to an inner core, rising through the inner bore of the conductor pipe. As the conductor pipe penetrates, the resulting earth compression adds to the viscous friction on the inner wall of the pipe, and becomes the major contributor to force delaying the driving.
In turn, as a result of the resistance caused by the core earth within the conductor pipe, there is a need, under current driving procedures, to drill out this plug at frequent intervals, causing expensive delays.
Some prior art attempts have been made to plug the bottom end of the conductor pipe with a shaped shoe so as to prevent the core from forming within the pipe. This, however, dramatically increases the force needed to displace an earth volume equal to the conductor pipe to a region outside the zone of the conductor pipe, and increases driving forces to the point that the pipe normally cannot penetrate deep enough to properly engage a consolidated soil sub-layer.